The invention is related generally to mobile communications and more particularly to a system and method for operating in an area and in a frequency spectrum in which existing communication systems are already operating.
The mobile communications industry has grown at a fast pace in the United States and even faster in some other countries. It has become an important service of substantial utility and because of the growth rate, saturation of the existing services is of concern. Contributing to this concern is the congestion of the electromagnetic frequency spectrum which is becoming increasingly severe as the communications needs of society expand. This congestion is caused not only by fixed microwave service or site (xe2x80x9cFMSxe2x80x9d) installations and Broadcast Auxiliary Services (BAS) but also by other communications systems including mobile communications systems. In the mobile communications industry alone, it is estimated that the number of world-wide mobile subscribers will increase by an order of magnitude in the next ten years. The radio frequency spectrum is limited and in view of this increasing demand for its use, means to use it more efficiently are constantly being explored.
Increasing demands for mobile radio service have put great strain on the frequency allocation process. This is particularly true for those frequencies currently felt to be uniquely usable for mobile radio; that is, the UHF band from roughly 300 to 3,000 MHz. Much of this precious spectrum is presently allocated to various kinds of services using receivers located at fixed sites. In recognition of the particular value of these frequencies for mobile radio service, the Federal Communications Commission (xe2x80x9cFCCxe2x80x9d) has proposed plans for relocating some of the fixed microwave service users to other frequency bands to make room for emerging technologies and personal communications systems particularly in the mobile services. Relocation may be a lengthy process taking place over many years. In the meantime, new mobile services may be licensed on a xe2x80x9cnot-to-interferexe2x80x9d and xe2x80x9cnot-to-claim interferencexe2x80x9d basis with respect to the incumbent systems.
Detailed study of the geographic distribution of the present microwave service licensees indicates that while in certain geographical areas these bands are almost fully allocated for use as fixed microwave services of a given type, there is still considerable usable frequency spectrum available in the geographical and frequency spaces between the interference zones of present microwave service installations.
For example, Electronic News Gathering (ENG) communication systems operate extensively in the United States. ENG employs a variety of auxiliary services to support basic television operation. Five major video auxiliary services, (studio transmitter links (STL), intercity relays (ICR), temporary fixed systems, van mounted transportable ENG systems and point of view systems), utilize a variety of frequency bands collectively known as the Broadcast Auxiliary Services (BAS) bands. The 1990-2110 MHZ band is used as the primary ENG band because of favorable propagation effects. ENG services include both mobile point of view and transportable ENG systems which provide mobility for news coverage throughout a geographic region. Due to limits on frequencies, time sharing of channels and the fact that multiple ENG receive sites and systems only exist in the largest television markets, there is still considerable usable spectrum available in the geographical spaces between the interference zones of the service installations.
As used herein, an xe2x80x9cinterference zonexe2x80x9d refers to a geographic region surrounding a system or site having priority over other systems in a designated frequency band operating in the electromagnetic spectrum within which harmful interference would be encountered from other systems operating in the same frequency band. The priority system may be mobile or fixed. Should the transmission by a second system interfere with the reception of a first xe2x80x9cpriorityxe2x80x9d system, the second system would be considered to be operating in the interference zone of the first system. Should the receiver of a second system receive harmful interference from the transmission of a first system, the second system would also be considered to be operating in the interference zone of the first system.
In a system that provides both transmission and reception, there are thus two interference zones and these two interference zones may differ. A xe2x80x9ctransmit interference zonexe2x80x9d is a minimal geographical region about the fixed system""s radio communication receiver system defined for a particular frequency band where transmission by a mobile transmitter system on that frequency band in that geographical region could cause harmful interference to the fixed system""s receiver system. A xe2x80x9creceive interference zonexe2x80x9d is a minimal geographical region about a fixed system""s radio communication transmitting system defined for a particular frequency band where a mobile receiver system in that frequency band within that geographical region could receive harmful interference from the fixed system""s transmitting system. The interference zone is specified by frequency bands and by three dimensions of geography.
Geosynchronous communications satellites over the United States have shared frequency spectrum with fixed microwave service ground towers from the beginning of the commercial space age. Such coexistence is practical because the satellite-to-terrestrial microwave geometry is fixed at a range of about 22,000 nautical miles and at elevation angles greater than about 20 degrees. This provides an acceptably large minimum angle between a geostationary satellite and the fixed ground microwave system. When the ground fixed microwave system follows category xe2x80x9cAxe2x80x9d antenna requirements in accordance with FCC regulations, and the satellite appropriately limits its flux density on the ground, the two systems can operate on a mutually non-interfering basis. Even though the two systems use common frequencies, the satellite service is not in the interference zone of the ground tower systems and vice versa.
This compatible operation of geostationary satellite systems with terrestrial microwave stations in a common frequency band is facilitated by their fixed positions, the large distance between them, and the directivity of their antennas. Because the satellite system does not move relative to the FMS, BAS or other systems with fixed sites, it will never be in a geographically interfering position with the terrestrial microwave stations. Such is not the case with mobile users. Because mobile users are capable of continuously changing position and their antennas are not directive, their operation could interfere with an existing station. This arrangement would not provide a suitable xe2x80x9cnot-to-interferexe2x80x9d system with the existing licensees. However, as discussed above, there exists a large amount of geographical regions and frequency spectrum that is unused between the interference zones of the existing stations.
Hence those skilled in the art have recognized the need for a suitably agile, intelligent communications system having mobile radio users that can coexist with service installations operating in fully allocated frequency bands. Those skilled in the art have also recognized the need for a communications system for mobile users that can coexist on a not-to-interfere and not-to-claim interference basis with fixed microwave and BAS service. The present invention meets this need and others.
Briefly and in general terms, the invention is directed to a communication system for permitting the communications of a mobile unit in proximity to a communications service having priority over the communications of the mobile unit. The mobile unit transmitters operate in the general region of fixed site receivers, or even mobile receivers, of another service which has priority while using the same frequency band as that priority service. The invention eliminates interference with the priority service receivers when the mobile units of the secondary system roam into the transmit interference zones of the priority service receivers.
In one aspect of the invention, the priority communication system includes a receiver for receiving signals in a frequency band. The invention further includes a mobile communications unit including a receiver and transmitter with the transmitter transmitting in the same frequency band. An interference zone is formed around the priority communications site such that if the mobile unit were to transmit in the frequency band of the priority system it would cause interference with the operation of the priority receiver. In order to eliminate interference with the priority system by the mobile unit, the present invention includes a proximity detection means for determining if the mobile unit is located in the interference zone. Once the determination has been made that the mobile unit is located in the interference zone, means are provided for eliminating the interference of the mobile unit with the priority service. Interference may be eliminated, or in other terms, the interference zone may be altered, by ceasing the operation of the mobile unit, changing the operating frequency of the mobile unit, reducing the transmitting power of the mobile unit or by other means well known to those in the art.
In a first preferred embodiment of the invention, a transmitter is located at the priority site, collocated with the antenna serving the receiver of the priority service at that fixed site. The transmitter continuously or periodically (say, several times per second) emits a warning signal at one or more frequencies at close to the same frequency as the priority receiver operates through the priority site receiver antenna, or through an antenna which has the same general coverage pattern as the antenna serving the fixed site receiver and which is collocated with that antenna as closely as possible. This warning signal is intended for the mobile unit. The power level for this periodic signal is set at a magnitude such that the mobile unit detects and measures its power magnitude and processes that measurement to determine if it is inside the priority site interference zone. The mobile unit ceases transmissions when it determines that it is inside the transmit interference zone, or adjusts the mobile unit transmission to a low enough level that it will not interfere with the priority service receiver.
In this same aspect of the invention, it may be that different mobile receivers of the mobile service may have different thresholds of detection and also different transmitter power levels. Each mobile unit of distinct design would then be required to measure the signal strength of the warning signal and knowing its own transmitter power level, determine if the mobile unit transmitter is inside the transmit interference zone. In this process, the mobile unit transmitter power level may be decreased to, in turn, decrease the size of the transmit interference zone. In some simple implementations of the invention, the measurement of the power level need not be made when the mere detection of the warning signal is all that is required to determine that the mobile unit is inside the transmit interference zone and that the mobile unit transmitter should be turned off or prevented from turning on to transmit if it is already off. In another aspect of the invention, the mobile unit may simple switch to a frequency that does not interfere with the operation of the priority service provider.
In a second embodiment of the invention, a warning signal receiver is collocated with the priority service receiver. This warning signal receiver shares the fixed site priority service receive antenna or uses its own antenna that has nearly the same coverage and pattern as the priority service fixed site antenna and is collocated with it as closely as possible. The mobile unit system is designed so that the mobile units continuously or periodically transmit warning signals which identify each mobile unit and which are received by the warning signal receiver collocated at the priority service fixed site. The warning signal receiver will continuously detect and track all mobile unit transmitted signals that are in range of the priority site but not so close as to interfere with the reception of the priority service signals.
The invention is also based on the physical principal that two different microwave signals at almost the same carrier frequency (i.e., within the allocated priority service frequency band) will follow the same propagation path and attenuate over that path almost the same amount as a percentage of the transmitted power. Thus, if the emitted power of the warning signal is set at a known level with respect to a mobile unit communications signal, the power received by the warning signal receiver collocated with the priority service receiver will, with suitable corrections for the slight difference in frequency, be an indication of the power that would be received by the priority service from a potentially interfering communication signal from that same mobile unit.
The warning signal receiver measures the received signal power of a mobile unit transmission so that when the signal power exceeds a certain level, a warning command is generated and transmitted from the priority site to the potentially interfering mobile unit to either turn off that unit""s transmitter, to reduce its transmitted power to a level that will not interfere with the priority service, or to change its transmissions to a non-interfering frequency. The received power level threshold at which the turn-off command would be sent would be selected such that the mobile unit communication channel transmissions (that may be at a frequency anywhere within the allocated priority service band) will not interfere with the priority service. If the mobile unit was commanded off, after a suitable period long enough to allow the mobile unit to leave the transmit interference zone, the mobile unit would resume transmission of the special narrow band signal and the process would repeat as above until the mobile unit is outside the transmit interference zone, at which point it would be allowed to turn on its communication channel.